This invention relates to power converters and, more particularly, to a high power-factor a.c. to d.c. power converter for use with motor drives and power supplies, the power-factor being defined as the ratio of the true power to the product of the voltage and current in a circuit, and a high power-factor being one which approximates or exceeds 0.9, with the maximum power-factor being 1.0.
A boost-type power converter is a well-known and practical choice for a high power-factor converter used in motor drive and power supply applications. This converter circuit has many advantages. One of these is realization of the line current waveform needed to comply with line harmonic standards, both present and anticipated. Another advantage is that it can provide a nearly full conduction angle. However, a boost converter requires the output voltage to always be higher than the peak input voltage. If a lower voltage is needed as, for example, in motor drives operating at lower speeds; then, the voltage reduction function must be performed separately which means additional converter cost and complexity. Boost-type converters must also have power-on input current surge limiting circuits, and output short-circuit current limiting circuits.
Buck-type power converters are also generally well-known in the art, and are frequently used in d.c. to d.c. converters. They can also used as a.c. to d.c. power converters. For example, buck-type a.c. to d.c. converters have been used in motor drives for household appliances as shown in co-pending U.S. Pat. application Ser. No. 339,502 which is assigned to the same assignee as the present application. A major limitation of buck-type converters is that the duration for which power can be extracted from a single-phase line supply (input conduction angle) decreases when the converter output voltage increases. This occurs, for example, in the spin mode of the washer drive. A consequence of this limitation is the cost of the power converter increases while the benefits derived from using it decrease, especially when a wide range of output voltages (motor speeds) is required.
At power levels below 200 Watts, a third type of converter, well-known as a flyback or buck-boost type of converter, is frequently used. An advantage of this converter is the realization of both a high conduction angle and output voltage control. However, this converter is not cost effective for higher power levels because it imposes excessive levels of peak currents and voltages on its power device. Further, flyback converters produce an output voltage of inverted polarity and require substantial filtering at both its input and output.
As a result of the foregoing, there is a need for a cost-effective and efficient a.c. to d.c. converter which can provide a full conduction angle, and thus a high power-factor, at any level of output voltage. Further, the converter must operate over a wide power range extending from a few tens of watts to several thousand watts.